Three months after Russian authorities seized their ship and arrested them, five Britons who were among the Arctic 30 group of environmental protesters have arrived back in London on a Eurostar from Paris, looking relieved and in good spirits.

Anthony Perrett, 32, said: "It was worth it. I think we brought the world's attention to the fate of the Arctic and that's difficult to do because it's so far north.

"All the science is telling us that if humanity carries on as it is doing, in 1,500 years the planet will be dead. I don't know how big a price you have to pay for that. The price we paid was jail, and Christmas away from home in Russia," he told the Guardian as he left Paris with fellow Greenpeace activists Alexandra Harris and Phil Ball, a crew member of the Arctic Sunrise ship, Iain Rogers, and freelance videographer Kieron Bryan, one of two journalists on the trip.

They took the train to London after flying into the French capital from St Petersburg, where they had spent the second part of their detention, a period on bail and then a wait for visas after being granted an amnesty earlier this month.

The sixth Briton, activist Frank Hewetson, was travelling to another country, according to Greenpeace.

Perrett, from Newport, south Wales, said: "I thought that at some point we were going to get out and at other times that we were going to get stuck in jail. My feelings fluctuated. There were highs and lows. The lowest point was when I ran out of things to read [in jail]. You look around and your mind begins to wander and you speculate what might happen to you because there's not a massive amount of due process there.

"We certainly didn't find any justice for our cause. The prison services were all very professional, but the judiciary as a whole seemed farcical at times. The charge of piracy was preposterous and I don't feel I did any hooligan act at any time.

"I will be carrying on the campaign for Greenpeace and that might take me back to the Arctic. I don't want to end up in jail again, but if that is what it takes, I am prepared."

They will be met by their families. Perrettwill arrive in Paris on Friday and then travel by Eurostar to London told the BBC Radio 4 Today programme the experience had been worth it.

"I hope we have got conversations started in Russia about drilling in the Arctic and raised the issue with Russian voters," he said.

Perrett suggested the prospect of a long term in prison was nothing compared to ensuring the continuing existence of humanity: "What price can you put on that?"

He said that while the physical and "aesthetic" prison conditions in which the protesters and journalists had been held were like those of a second world war concentration camp, the guards had been professional and had not treated them like prisoners of war.

Asked whether he would go back to Russia for Greenpeace, Perrett said: "I don't think that they are planning to go back to Russia any time soon, to be honest".

He called the experience "a long hundred days", adding: "I am eager to get back to Wales and sleep in my own bed and get back to work."A spokesperson for Greenpeace said: "After 102 days it's great to have them on the way back home. We've spoken to them and they're excited to be coming back. It is a relief to their families, who have gone through a difficult time."

The group was detained in September for protesting against an Arctic offshore oil rig owned by the Russian gas giant Gazprom. They were initially charged with piracy and held in jail for two months before being given bail and then granted amnesty.

Harris said she thought the Russian government let them go to avoid global criticism in the lead-up to the 2014 Sochi Winter Olympics.

"I think it was the easy way out for Russia, to get rid of us before the Olympics began and before there's a big PR pressure from Greenpeace and the rest of the world," she said.

Harris said the fact they were given an amnesty, despite doing nothing wrong, was "not ideal".

"But I'm not in a position to refuse it, I can't have seven years hanging over me," she said of the jail penalty she was facing.

The activist said she would not rule out a similar protest again, despite slamming the appalling conditions she faced in custody.

"The conditions are really bad, it's cold, the food is inedible – it's not good," she said.

Harris said knowing her 29 fellow activists were going through the same experience helped her to weather the ordeal.

She said the Arctic 30 had been treated better than Russian prisoners.

"Because the world's watching us and they're scared of what we're going to say now," she said.

"There was no physical violence towards me but it was torture – we spent two months in a Russian jail cell and 100 days detained for a crime we didn't commit.

"It was obscene, a complete overreaction on the part of Russia, and we should never have been there."